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In what is being called the 2017 Great American Eclipse, about all of North America was swathed in different shades of darkness on August 21, 2017 in the event of a total solar eclipse (Fig 1). The total solar eclipse was viewed by people in North America, and a partial eclipse was visible to people in parts of South America, Europe and Africa.

The path of totality, or the line sketched by the 2017 Great American Eclipse covering narrow regions in total darkness passed through areas of 14 American states. To be in the path of totality in a total solar eclipse is when an observer views the eclipse when the Moon totally covers the Sun in such an angle such that for the observer the particular region is shrouded in darkness for a short duration of time.

A solar eclipse occurs when the Moon passes in between the Earth and the Sun such that the alignment blocks sunlight for certain regions of the Earth due to the Moon’s shadow and can last up to 3 hours for a particular region. A total solar eclipse, as is this one, is one where the Moon blocks all of the light of the Sun for a certain region of the Earth as the path of totality. A solar eclipse is a unique coincidence, as although the Sun is about 400 times larger than the Moon is, the Moon is also incidentally about 400 times closer to the Earth than it is to the Sun, enabling a total solar eclipse.

The longest period in the Great American Eclipse when the Moon blocked sunlight completely was 2 minutes and 40 seconds.

The US last witnessed a total solar eclipse in 1979. A total solar eclipse however, occurs somewhere on the Earth roughly once every 18 months and is problematic for observers because most of the Earth’s surface is covered by oceans. One’s position of viewing an eclipse is a significant determinant in how one experiences an eclipse. In an interesting coincidence, since the International Space Station goes around the Earth every 90 minutes, the astronauts aboard had the opportunity to observe the eclipse thrice. However, they were not able to see the complete total solar eclipse but were able to see only a partial solar eclipse (NASA, 2017).

Fig 1: The Areas covered by the Moon’s shadow during the 2017 Great American Eclipse
Source: NASA

Apart from looking at the total solar eclipse when an observer is within the ‘totality’, or when the Sun is totally covered by the Moon for an observer in the path of totality, it is highly unsafe to look directly at a solar eclipse and it may damage the human eye.

Instead, instruments called eclipse glasses are used, which are special-purpose solar filters, which very much resemble conventional glasses. Conventional sunglasses though, are not suitable for using during a total solar eclipse, as they transmit thousands of times more sunlight than eclipse glasses. It is also important that a user check his eclipse glasses for certification or damage.

It is only when the Moon completely covers the Sun and there is pitch darkness that one can remove one’s eclipse glasses, although even here discretion is advised. Eclipse glasses are compulsory if the observer is outside the path of totality of the total solar eclipse. A special solar filter is advisable even for cameras, although during the totality, one can remove the solar filter to photograph the Sun’s outer atmosphere – the corona. One can also capture the eerie light effects across the Earth’s landscape as the Moon transits during the total solar eclipse.

Significance for Science

The focal point of interest for scientists during a total solar eclipse is to study the corona, or the Sun’s outer atmosphere that is visible as the Moon obscures most of the Sun in a total solar eclipse such that only the corona is visible (Fig 2). The instrument that scientists use in order to study the Sun’s atmosphere is called a coronagraph, which is an attachment in a telescope that obscures the otherwise intense brightness of the Sun.

The coronagraph makes it possible to study the corona even when there isn’t a total solar eclipse, but the event of a total solar eclipse makes it easier for scientists to observe apart from the corona, even the Sun’s lower atmosphere as it becomes more clearly visible using precise instruments. Until the 1930s, scientists would wait until the event of a total solar eclipse to study the corona. The study of the corona during a solar eclipse with the help of a spectroscope was what enabled the French astrophysicist Pierre Jules César Janssen to first identify in 1868 what is now known as Helium (G. Guglielmi, 2017).

Fig 2: The Sun’s Corona (or outer atmosphere) as captured by a satellite during a total solar eclipse
Source: NASA, flickr

The Great American Eclipse presented a great opportunity for scientists to make scientific observations as the path traced by the shadow of the Moon on Earth lasted for about 90 minutes. The study of the Sun’s atmosphere allows scientists to make better observations of phenomena such as solar winds, which are charged particles and magnetic fields discharged by the Sun that are deflected by the Earth’s magnetic field, sometimes leading to aurora lights at the Earth’s poles.

The study of the Sun’s lower atmosphere and how particles move in it can also help scientists better understand why the corona has higher temperatures than the Sun’s surface, which seemingly defies logic. This study along with an investigation into the magnetic field structure of solar winds were part of the National Aeronautics and Space Administration’s (NASA’s) project during the 2017 Great American Eclipse (NASA, 2017).

A total solar eclipse can also help scientists in understanding how coronal mass ejections, or solar flares work in terms of the study of the composition of their radioactivity. This can offer a great deal to scientists in understanding sunspots, including their migration patterns in the Sun’s coordinates.

A total solar eclipse also offers scientists to study the planet Mercury, closest to the Sun. The study of Mercury during a total solar eclipse in infrared light can reveal its temperature (P. Chandra, 2017). The duration of a total solar eclipse can also help in studying the diffraction of light from distant stars within the Moon’s alignment. This was helpful in studying the warping of light by extremely strong gravitational fields and can help us in understanding more about distant objects.

The light gathered by telescopes is a significant factor in many calculations made by astrophysicists and scientists in evaluating interstellar phenomena, and a total solar eclipse is one such event in the neighbourhood where the scientific community is activated, especially if the eclipse occurs over landmasses with telescopes and observatories.

Lunar Eclipse Predicted for January 2018

Although the solar eclipse for India is predicted to occur in 2019, a lunar eclipse has been predicted by NASA for January 2018 (NASA, 2017). The lunar eclipse, expected to be observable from all of India, is expected to occur on January 31, 2018. A lunar eclipse is caused by Earth’s shadow on the Moon and can sometimes make the Moon reddish in its hue. If one is in Delhi between 18:00 to 19:37 hours on January 31, 2018, it is possible that one might be able to observe a reddish Moon in the sky, if the predictions are correct.